Seismic interpretation of the Southern Uplands terrane

[摘要] The Southern Uplands Network (SUN) project comprised 5 wide-angle seismic profiles recorded in the Southern Uplands of Scotland using quarry blast sources. Four profiles trend N-S across regional strike and the fifth is parallel to it. A sixth profile was recorded, in the north of the Midland Valley, along the axis of the Strathmore Syncline to establish the basin-basement relationships in that area. A large and good quality dataset was acquired which was then processed using the available facilities and computer software to obtain the final models by deploying the appropriate interpretation methods. P-wave velocities of 5.25-5.65 km/s were modelled for Lower Palaeozoic sediments in the Southern Uplands. P-wave velocities of 4.3-4.5 km/s were assigned to minor Upper Old Red Sandstone deposits. Crystalline basement of 6.0-6.1 km/s was detected beneath the NE Southern Uplands, suggesting continuation of Midland Valley basement southwards at a depth of 2.8-3.2 km. However, the SUN suggests that this basement does not extend along strike in the Southern Uplands towards the SW, where an "intra-sedimentary" refractor (5.8 km/s) is detected, perhaps of Ordovician age, at 1.0-2.0 km depth. It may be the missing forearc proposed by some authors. Throw on the Southern Uplands Fault is varied. In the NE it downthrows northward; in the SW downthrow southward is inferred. Three E-W trending faults (the Leadhills Line, Kingledores Fault and Hartfell Line) offset crystalline basement beneath the Southern Uplands. Previously these were interpreted as tract bounding faults, characteristic of an accretionary prism. SUN reveals a more important tectonic role for them, perhaps being locations for amalgamation of terranes forming the Southern Uplands. SUN did not image any accretionary prism tract bounding faults. The observed faults suggest a stepped, rather than dipping, crystalline basement underlying the Southern Uplands. These faults also offset the intra-sedimentary refractor. Previous work established Midland Valley basement as essentially flat, with faults mapped at surface soling out at detachments above this basement, and that it extends south unaffected by the Southern Uplands Fault. SUN shows this basement to be offset by faults in the southern Midland Valley (e.g. Henshaw, Pent-land, Kerse Loch and Southern Uplands Fault). In the northern Midland Valley NW-SE trending faults also offset basement causing a step-like pattern. Top crystalline basement exists at 2.0-4.9 km depth here being shallower in the NE. SUN confirmed velocity ranges already established for the Midland Valley upper crust. It is argued that the SUN provides strong evidence against the Southern Uplands being a complete accretionary prism. The following models may be invoked: [1] Accretionary prism slice thrust over Midland Valley-type crystalline basement. [2] Juxtaposition of a number of terranes along major faults inteipreted by SUN. [3] A thrust stack.

[发布机构] University:University of Glasgow

[关键词] QE Geology